In Browse Mode, when adjacent objects have anchors defined to extend, a proportional resizing would avoid an object to "eat" another.
In other words, allowing multiple adjacent objects to be extensible.
Example taken on Layout Mode where you can, since v12, resize multiple objects simultaneously and where a proportional resizing is applied.
This is also a longstanding wish of myself. I uploaded this feature request one day after the anchoring/resizing feature appeared in FM. I don't recall it exactly, but that was a long time ago.